[multibody] MakeMobilizerForJoint() can now create Frames

[sherm1]

Allows a joint to choose mobilizer F and M frames that are distinct from the joint's parent and child frames Jp and Jc. Demonstrates the capability by moving the Weld mobilizer F frame to be coincident with the M frame, even if Jp and Jc are not coincident, and strips the Weld mobilizer of its ability to model the F≠M case. Requires reaction force computation to understand whether Jc and M are the same because if not the reaction force must be shifted from the origin Mo of M to the origin Jco of Jc. Reverse welds must also be accommodated.

Existing tests in multibody_plant_test.cc already look at reaction forces for a variety of welds, reversed and with a general X_JpJc. Those fail without the reaction force changes here.

This does not include

• modifications to other mobilizer types
• changes to algorithms that permit taking advantage of the available speedup
• any of the M-frame work in [multibody] Experiment using M frame for Inverse Dynamics, for timings/sanity check #22253 (that is orthogonal to moving the frame around)

This is the first PR in a series leading to MultibodyPlant taking advantage of optimal mobilizer frame choice for faster kinematics.

Notes for Release engineer

This is a change to the protected Joint API (added a parameter to pure virtual MakeMobilizerForJoint()). That only affects people who have written new Joint & Mobilizer types. If there are any non-Drake Joint extensions, they will need to add that parameter to the signature, although it can be ignored. Per our policy as stated in the Joint class header, the Joint protected API is subject to change without deprecation. However, we promise to note when changes have been made.

Resolves #22648

---

This change is 

[sherm1]

+@amcastro-tri for feature review, please

Per earlier f2f, although MbT has inverse dynamics that returns M-frame reactions, I don't see a public MbP API that does so. The one that is there just returns taus. So as best I can tell the only place that cares if Jc != M is CalcReactionForces() which does promise to return reactions at Jc and expressed in Jc. PTAL

Reviewable status: LGTM missing from assignee amcastro-tri, needs platform reviewer assigned, needs at least two assigned reviewers

[sherm1]

Reviewable status: 1 unresolved discussion, LGTM missing from assignee amcastro-tri, needs platform reviewer assigned, needs at least two assigned reviewers

---

multibody/tree/weld_mobilizer.h line 54 at r1 (raw file):

  // is always the identity transform since F==M by construction.
  math::RigidTransform<T> calc_X_FM(const T*) const {
    return math::RigidTransform<T>();

BTW @amcastro-tri where this is headed: this function will be replaced by inline operators that apply or compose with X_FM rather than returning it literally. In the case of weld those will be no-ops and generate no code.